In the drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. Under the miniaturizing trend, the lithography has achieved formation of finer patterns by using a light source with a shorter wavelength and by a choice of a proper resist composition for the shorter wavelength. Predominant among others are positive resist compositions which are used as a single layer. These single layer positive resist compositions are based on resins possessing a structure having resistance to etching with chlorine or fluorine gas plasma and provided with a resist mechanism that exposed areas become dissolvable. Typically, the resist composition is coated on a patternable substrate and exposed to a pattern of light, after which the exposed areas of the resist coating are dissolved to form a pattern. Then, the patternable substrate can be processed by etching with the remaining resist pattern serving as an etching mask.
In an attempt to achieve a finer feature size, i.e., to reduce the pattern width with the thickness of a resist film kept unchanged, the resist film becomes low in resolution performance, and if the resist film is developed with a liquid developer to form a pattern, the so-called “aspect ratio” (depth/width) of the resist pattern becomes too high, resulting in pattern collapse. For this reason, the miniaturization is accompanied by a thickness reduction of the resist film (thinner film). On the other hand, with the progress of the exposure wavelength toward a shorter wavelength, the resin in resist compositions is required to have less light absorption at the exposure wavelength. In response to changes from i-line to KrF and to ArF, the resin has made a transition from novolac resins to polyhydroxystyrene and to acrylic resins. Actually, the etching rate under the above-indicated etching conditions has been accelerated. This suggests the inevitableness that a patternable substrate is etched through a thinner resist film having weaker etching resistance. It is urgently required to endow the resist film with etching resistance.
Meanwhile, a process known as multilayer resist process was developed in the art for processing a patternable substrate by etching. The process uses a resist film which has weak etching resistance under the etching conditions for the substrate, but is capable of forming a finer pattern, and an intermediate film which has resistance to etching for processing the substrate and can be patterned under the conditions to which the resist film is resistant. Once the resist pattern is transferred to the intermediate film, the substrate is processed by etching through the pattern-transferred intermediate film as an etching mask. A typical process uses a silicon-containing resin as the resist composition and an aromatic resin as the intermediate film. In this process, after a pattern is formed in the silicon-containing resin, oxygen-reactive ion etching is carried out. Then the silicon-containing resin is converted to silicon oxide having high resistance to oxygen plasma etching, and at the same time, the aromatic resin is readily etched away where the etching mask of silicon oxide is absent, whereby the pattern of the silicon-containing resin is transferred to the aromatic resin layer. Unlike the single layer resist film, the aromatic resin need not have light transmittance at all, allowing for use of a wide variety of aromatic resins having high resistance to etching with fluorine or chlorine gas plasma. Using the aromatic resin as the etching mask, the patternable substrate can be etched with fluorine or chlorine gas plasma.
Typical resins used in the bilayer resist process are polysilsesquioxanes. In chemically amplified resist compositions of negative type, polysilsesquioxane having side chains exhibiting solubility in alkaline developer is typically used in combination with crosslinkers and photoacid generators. In chemically amplified resist compositions of positive type, polysilsesquioxane having acidic side chains protected with acid labile groups is typically used in combination with photoacid generators. In general, the polysilsesquioxanes are prepared through condensation reaction of trifunctional silane monomers. Since an ordinary synthesis method yields a polysilsesquioxane product with a noticeable amount of silanol groups left therein, compositions containing the same suffer from shelf instability.
Cage silsesquioxanes are typical of silsesquioxanes (SSQ) which are substantially free of silanol groups, that is, have a degree of condensation of substantially 100%. In general, polyhedral oligomeric silsesquioxanes of 6 to 12 monomer units are known and abbreviated as POSS, of which an oligomer of 8 monomer units (referred to as octet) is relatively readily available. Patternable material using POSS compound is described in US Patent Application 2004-0137241 A1 (JP-A 2004-212983). A resist composition is synthesized by starting with a POSS compound having hydrogen as substituent groups and introducing side chains therein.
For SSQ compounds having a degree of condensation of 100%, exemplary frameworks include those of 6 to 12 monomer units shown below.

Herein, each apex denotes a silicon atom having one substituent group, and each side denotes a Si—O—Si linkage.
Meanwhile, condensates (SSQ) of alkyl-bearing trifunctional silicon compounds are attractive as organic-inorganic composites and have found a wide range of actual application, for example, as base polymer in the bilayer resist process. Regrettably, they lack shelf stability because hydrolyzable silanol or alkoxy groups are left behind as described above. An improvement in shelf stability is desired. With respect to the octet (POSS compound) having a degree of condensation of 100% having the ultimate structure, shown below, which is expected as the material capable of solving shelf instability, several reports were already published and their preparation methods are known. In most of these methods, equilibration reaction is carried out in the presence of a base catalyst over a long time. For utilizing the high crystallinity of the octet having a degree of condensation of 100%, an appropriate solvent is selected as the reaction medium so that the octet may be crystallized and isolated from the system. In this case, although a shift of equilibrium occurs as a result of the product or octet having a degree of condensation of 100% being crystallized and removed out of the reaction system, condensation generally proceeds to a full extent even under the condition that water is co-present in the reaction system. Then a starting substance having a relatively small substituent group enough to form an octet must be selected. Since isolation and purification depends on a level of crystallization, the substituent group that can be introduced is restricted. Complex steps are necessary to introduce substituent groups of different type.

In the other reported method, once a SSQ polymer which is in an incompletely condensed state, but has a higher molecular weight as prepared by a prior art technique is taken out, it is converted into a cage structure using a large amount of base catalyst (see JP-A 2003-510337). It is unknown to apply this method to those monomers having less condensable bulky side chains which cannot be once taken out as a polymer.
On the other hand, it is known in the art that SSQ materials are prepared by carrying out base catalyzed condensation reaction at a high temperature above 100° C. for a prolonged time. The inventors discovered in JP-A 2004-354417 that polymeric compounds having a relatively high degree of condensation are obtained by carrying out the condensation reaction in two stages, that is, a first stage of hydrolysis to form a partial hydrolyzate of silicon, followed by concentration and isolation for removing the alcohol by-product and extra water, and a second stage wherein the base catalyst is used in a large amount of at least 10% based on the monomer units.